A large, sustained plasma volume expansion (PVE) is a necessary feature of normal, successful pregnancy. Suboptimal PVE is associated with adverse fetal outcome, and is a component of preeclampsia. The goal of this project is to determine the primary mechanism of the normal gestational PVE, so that ultimately, complications of pregnancy associated with failure to PVE can be prevented. This proposal is motivated by several marked similarities between the renal and hemodynamic responses to normal pregnancy, and those seen in states of inappropriate PVE, such as nephrosis or cirrhosis. The primary renal mechanism in pathological sodium retention is a failure of natriuretic mechanisms that rely on cGMP as second messenger, due to increased cGMP hydrolysis by phosphodiesterase (PDE). Here, we will test the hypothesis that selective upregulation of the renal tubular PDE system (particularly the inner medullary collecting duct, IMCD) occurs in pregnancy and causes widespread refractoriness to natriuretic stimuli, such as acute volume expansion and administered atrial natriuretic peptide, leading to PVE. We will also characterize the isoforms and location in the kidney of the PDEs involved in the renal response to normal P and determine whether increased renal PDE activity in pregnancy results from translational or posttranslational events. We will investigate how angiotensin II and renal nerve activity (major renal sodium retaining systems) are involved; we will test the "underfill" hypothesis of gestational PVE and we will determine whether the maternal hormones progesterone, prolactin or relaxin are primarily responsible for the increased renal PDE activity and PVE of P. Finally, we will chronically and locally inhibit the relevant PDE(s) during pregnancy to establish the impact on maternal hemodynamics and fetal outcome. All studies will be in rat, using a combination of in vivo techniques to measure renal sodium handling capacity and in vitro techniques on isolated glomeruli, proximal tubules and inner medullary collecting duct cells, to include enzyme activity assays, Western blot, immunocytochemistry, Northern blot or RNAse protection assay and in situ hybridization. [unreadable] [unreadable]